Field of the Invention
The present invention relates to a device and method that are intended to be used to perform electron-spin-resonance (ESR) spectroscopy with a greater sensitivity than in the prior art.
ESR spectroscopy in particular has applications in structural biology, solid-state physics and archaeology.
Prior Art
Current spectrometers have a sensitivity that typically allows them to detect 109 spins in one second of integration time.
The purpose of an ESR spectrometer 10 is to detect and characterize electron spins in a given sample 1. To do this, the sample 1 is inserted into an electromagnetic resonator 2 having a resonant frequency ωr in the microwave domain (typically from 5 to 10 GHz) and a quality factor Q (see FIG. 1).
A magnetic field BO is applied to the sample 1 in the direction of the arrow 3 by a coil or any other type of device (not shown) for creating a magnetic field, in order to bring the transition frequency of the spins (given by ωs=γB0, γ being the gyromagnetic ratio of the spins) into resonance with the resonant frequency ωr.
This magnetic resonance is detected by virtue of a microwave signal that is injected into the cavity at the frequency ωr by a device 4 and collected, on exiting the sample 1, by a device 5, in order to be amplified by an amplifying device 6.
The transmission T of this signal is decreased when ωs=ωr, because the spins then absorb this microwave signal, this manifesting itself in the appearance of resonant dips in the curve T(B0). The frequency and width of the dips, and their relative amplitude, allow a number of pieces of information on the sample 1 to be extracted.
This spectrometer operating mode is called continuous-wave EPR spectroscopy.
Another widely used operating mode (referred to as “pulsed EPR”) that implements a device such as that in FIG. 1 consists in using sequences of brief microwave pulses that cause the spin to rotate by a well-defined Rabi angle.
One of the most used sequences is called the spin-echo sequence. It consists in applying a first pulse that causes the spins to rotate by an angle of π/2, followed, after a time τ, by a second pulse that causes a rotation of angle π. Spins meeting the condition of resonance ωs=ωr then emit an echo signal at the time 2τ.
The dependence of the amplitude of this echo signal on certain parameters (magnetic field, angle, time τ between the pulses, etc.) contains all the information able to be extracted from the sample.
Detection of the spin-echo signal is therefore essential and is the keystone of electron-paramagnetic-resonance spectroscopy.
The maximum power P transmitted during a spin echo to the cable connecting the resonator 1 to the detection chain 6 is given by a simple formula: P=ℏωrN2g2/κ, N being the number of spins contained in the sample, g being the “coupling constant” of a spin to the microwave field of the cavity, and κ=ωr/Q the damping ratio of the field in the microwave resonator.
The duration of the echo pulse depends on the sample, but is given approximately by the “free-induction time” T2*.
The sensitivity of the spectrometer may then be quantified by the minimum number of spins Nmin detectable with a signal-to-noise ratio of 1 during a spin echo.
This number clearly depends on the amount of noise added by the first amplifier of the microwave detection chain 6, which is characterized by its noise temperature TN. The degradation of the signal-to-noise ratio during the amplification is given by the number of noise photons added by the amplifier, which is given by n=1(eℏωr/κTN−1).
Conventional microwave amplifiers all operate in the limit where κTN>>ℏωr and hence n≈κTN/ℏωr.
The minimum number of spins detectable by the spectrometer in one spin echo is then calculated by setting the number of photons emitted by the spins during an echo (T2*P/ℏωr) equal to the number of noise photons emitted during T2* in a passband 1/T2* (i.e. n), this implying that
      N          m      ⁢                          ⁢      i      ⁢                          ⁢      n        ≈            coth      ⁡              (                              ℏ            ⁢                                                  ⁢            ω            ⁢                                                  ⁢            r                                2            ⁢            kT                          )              ⁢          1      g        ⁢                            n          ⁢                                          ⁢          κ                          T          2          *                    (the coth first term being due to the equilibrium polarization of the spins at the temperature T).
In a conventional spectrometer, the resonator 2 is a metal box that contains the sample 1, and the microwave field inside the resonator 2 occupies a volume of about λ3, λ being the wavelength at the frequency ωr. This results in a typical coupling constant of g=2π×5 mHz. The amplifier 6 at room temperature adds about n=103 noise photons. The quality factor is typically 2000.The polarization factor of the spins at room temperature is
      coth    ⁡          (                        ℏ          ⁢                                          ⁢          ω          ⁢                                          ⁢          r                          2          ⁢          kT                    )        ≈            10      3        .  This leads to a sensitivity of Nmin≈1013 detectable spins in one spin echo for a conventional spectrometer at room temperature.
Recently, new types of spectrometers have been developed, the spectrometers being based on microresonators 102. It is a question of microwave resonators fabricated from thin metal films (see FIG. 2).
The volume of the mode may then be much smaller than λ3 and hence the coupling constant may be much higher, reaching g=2π×1 at 20 Hz. Microresonators 102 have been produced with what is called a “loop-gap” geometry, with thin films made of normal metals (see for example the article by Y. Twig, E. Suhovoy, A. Blank, Rev. Sci. Inst. 81, 104703 (2010)).
Microresonators 102 have also been produced in a coplanar waveguide geometry (see the example of FIG. 2) in which the coplanar waveguide is made of superconductive metal, as for example described in the article by H. Malissa, D. I. Schuster, A. M. Tyryshkin, A. A. Houck, S. A. Lyon, Rev. of Sci. Instr. 84, 025116 (2013).
Working at 4 K in order to increase spin polarization, and moreover using better amplifiers cooled to 4 K, for which as few as n=10 to 20 noise photons are obtained, sensitivity has been increased to a record value of Nmin≈107 spins in one spin echo, which is the highest sensitivity that has been published in the literature to date (see the article by A. J. Sigillito et al., Appl. Phys. Lett. 104, 222407 (2014)). It would however be desirable to be able to further increase such a sensitivity.